The object of the invention is a device that allows measuring blood sugar levels in mammals—primarily humans—by measuring the refraction of electromagnetic radiation from the skin or body tissue, without the need for invasive sampling, such as penetrating or pricking the skin. The device uses the blue light portion of the visible spectrum and near-infrared (IR) radiation as a source of electromagnetic radiation. The device falls within the field of medical diagnostics and is essentially designed to be portable, so that the users can wear it on their wrists, for example. The signals of reflected light/radiation measured by the sensor unit are filtered by the frequency filtering set and mathematically processed in order to Calculate the current blood sugar level. The accuracy of the measurement result of the device is comparable to the results obtained with the standard method of measuring by blood sampling. A method for measuring of blood sugar level using said device is also disclosed.